Several dual frequency RFID integrated circuits have been proposed. In particular for the passive devices, the power supply has to be defined and a corresponding integrated circuit has to be arranged accordingly. The power supply generally comprises at least a power generator. In case of two power generators respectively associated to the induced voltage in the resonance circuit of the HF antenna-coil and to the induced voltage in the UHF antenna, a power management is generally provided and the integrated circuit is designed for implementing such a power management.
A dual frequency HF-UHF identification device of the prior art is shown in the FIG. 1, which is a simplified design of the dual frequency tag described in the patent application US 2005/0186904. This dual frequency HF-UHF identification device 2 comprises a HF part 4 and a UHF part 6. The HF part 4 is formed by an HF antenna-coil 10, a resonance capacitor 8, a HF rectifier 12 and an analog HF front end 14 (AFE_HF). The UHF part 6 is formed by a UHF antenna 16, a UHF rectifier 18 and an analog UHF front end 20 (AFE_UHF). The device 2 also comprises a logic unit 22 and a non-volatile memory 24 (NVM), the logic unit 22 been connected either to the analog HF front end 14 or to the analog UHF front end 20 through multiplexers (not shown on FIG. 1) arranged in a first part of the logic unit. The HF rectifier is a diodes rectifier which rectifies the induced voltage in the resonance circuit formed by the antenna-coil 10 and the resonance capacitor 8. On its two input terminals V1a and V1b, the HF rectifier alternatively receives positive and negative induced voltages. This HF rectifier generates at its output a first supply voltage VHF. In a variant, such a HF rectifier can be associated to a voltage amplifying circuit for generating the first supply voltage. The UHF rectifier is formed by a charge pump linked to the two wires of the UHF antenna 16. This UHF rectifier receives at its two input terminals V2a and V2b positive and negative induced voltages and is arranged to provide at its output a second supply voltage VUHF.
Further, the device 2 comprises a combined power and mode management formed by a mode detection unit 26 and a switch 30. The mode detection unit senses an induced voltage in the HF resonance circuit (for example the voltage V1a) and delivers a mode signal DET the value of which indicates if the HF antenna-coil detects the receipt of an HF electromagnetic field in the resonance range of the HF resonance circuit (e.g. 13.56 MHz). The mode signal controls the control gate of the switch 30 through a first control line 31 and the logic unit 22 through a second control line 32. The switch is arranged between the UHF rectifier output and the supply line 28 providing a supply voltage Vsup to the electronic circuit. The operation of the power management is as follows:
a) When unit 26 detects at least a given voltage in the HF part of the power generator (in particular an induced voltage in the HF resonance circuit or alternatively at the output of the HF rectifier), the output of unit 26 is set to a first logical value (e.g. ‘1’) and the mode signal DET indicates that there is an incoming HF electromagnetic field. The switch 30 is arranged so that, when the mode signal is set to the first logical value, this switch opens (‘OFF’ position) and thus the UHF power generator is not active, whether the voltage VUHF is zero or not. The integrated circuit of device 2 is thus only powered by the HF part so long the detected voltage by the mode detection unit is at least equal to said given voltage. Further, the logic unit receives this mode signal and activates the HF protocol and associated circuit portions, in particular the multiplexers are put in a first state wherein only HF demodulated signals are transmitted to the logic unit (no UHF signal is transmitted).
b) When unit 26 does not detect at least said given voltage in the HF part of the power generator, the output of unit 26 is set to a second logical value (e.g. ‘0’) and the mode signal DET indicates that there is no incoming HF electromagnetic field. The switch 30 is turned on and is thus closed, allowing the UHF part of the power generator to power the integrated circuit if an induced voltage is generated in this UHF part. Further, the logic unit receives this mode signal and activates the UHF protocol and associated circuit portions, in particular the multiplexers are put in a second state wherein only UHF demodulated signals are transmitted to the logic unit (no HF signal is transmitted).
The device 2 is thus arranged so as to give priority to an incoming HF field. An opposite choice can be made by giving priority to the UHF field in a similar manner as described here above. However, the alternative giving priority to the HF field is preferred, as taught by the US patent application in question, in order to give priority to the writing of data in the NVM memory. It is to be noted that this document teaches that the voltage generated by the UHF part of the power generator is too low for erasing and programming an EEPROM or a FLASH memory. Thus, even if the switch is arranged between the HF rectifier output and the supply terminal Vsup of the storage capacitor, the teaching is to select the HF mode as soon as a given induced voltage is detected in the HF resonance circuit.
The device 2 has many drawbacks. First, there is a mode selection allowing only a HF or a UHF communication, but not both simultaneously. Then, if a given activity level is detected in the HF range, generally a low one for sensitivity reasons, the UHF communication is deactivated so that no communication in the UHF range can occur. If an UHF communication is running, the detection of the given voltage in the HF part will stop such a communication. Finally, the power management does not allow a HF field provided to the device 2 to participate to the power supply of this device for a UHF communication, and inversely. This is a big disadvantage of the device 2.
A passive device with multiple energy harvesting and communication channels is described in the patent application US 2009/0117872. This document illustrates one general embodiment at its FIG. 1A with a more detailed electronic design given in FIG. 2A. The passive device includes two antennae which are respectively coupled to two 1-stage half wave diode rectifiers which are arranged in series. The output capacitors of these two rectifiers are thus connected in series and a further common capacitor also stores energy from the two rectifiers in order to provide a DC power to the passive device. This implementation is particular. First, it is to be noted that the first antennae can provide energy to both output capacitors of the two rectifiers when the second antenna can provide energy only to the second output capacitor. Further, it is to be noted that each antenna is coupled to the electronic circuit through an entry capacitor which is connected in series with the antenna. Several problems occur with such an electronic design for a multiple energy harvesting. First, the capacitive coupling is generally used for frequencies largely above the HF frequency normally selected at 13.56 MHz, in particular for the UHF frequency range. However, the kind of rectifier which is taught in the document US 2009/0117872 is much more appropriate to LF or HF signals. Practically, it would not be reasonable to use such a power generator with a HF signal received by the first antenna and a UHF signal received by the second antenna. The coupling capacitor would be very big and with a lot of leakage current due to its size. To have approximately the same impedance at the entries of both rectifiers, the size of the entry capacitor for the HF channel should be 200 times larger than the one for the UHF channel. Thus, the HF entry capacitor would not be easily integrated in an integrated circuit because it would need a too large space. Further, when the frequencies of the two signals respectively received by both antennae are different, destructive interference can occur with the described power generator so that energy will be lost.
Other particular embodiments are described in FIG. 1B to FIG. 1I of the document US 2009/0117872 but the teaching seems to indicate that these are particular cases with special functioning. For examples, with reference to the embodiment of FIG. 1G, it is written that the output currents of both rectifiers have to be the same and, with reference to the embodiment of FIG. 1H, it is written that each of the output voltages of both RF rectifiers is about equal to the input voltage of a power regulator, i.e. the output voltages of both RF rectifiers are about equal. Most of the time, such a situation will not occur with two different signals respectively received by both antennae. Further, one aim of a multiple energy harvesting is to be able to harvest energy from one source or the other or from both with different generated voltages.